Whereas, generally, a camera is focused to a point in a scene being imaged by the camera along its optic axis, some cameras enable a user to point the camera in a desired direction, but focus the camera to one of a plurality of off axis focus areas of the scene. Locations of the off axis focal areas are predetermined and fixed relative to the camera's optic axis. When imaging a scene, the camera can be pointed along a direction that provides a desired composition of features in the scene with a given region of interest (ROI) located at one of the fixed off axis focal areas. The focus may then be locked to the off axis focal areas to acquire an image in which the given ROI is in focus for the desired composition. A modern camera may typically be configured with between 3-11, and possibly as many as 51, preset, off axis focusing areas to which the camera focus can be automatically locked.
Various methods and devices are known in the art for focusing a camera whether focusing is preformed for on-axis or off-axis areas in a scene being imaged by the camera. For many cameras focusing is performed responsive to contrast of an image of a scene that the cameras are used to image. Images that are in focus are sharper and show detail more clearly than images that are not in focus. Sharper, in focus images comprise higher spatial frequency components than images that are blurred and out of focus. Typically, focusing a camera on a scene by contrast involves acquiring an image of the scene for each of a plurality of “trial” positions of a focusing lens or lens system of the camera. Each image is sampled and the samples for an image are used to determine spatial frequencies in the image. A trial position of the focusing lens or lens system for which the corresponding image exhibits highest spatial frequencies is considered to be an in focus position of the focusing lens. Focusing by contrast is generally relatively computation intensive and slow. Often, to reduce computational complexity and time to focus, the number of trial positions is reduced and/or sampling is performed with decreased pitch. However, reducing the number of trial positions or sampling pitch, generally reduces accuracy of focusing.
Various triangulation methods and implementing apparatus are also known in the art and used for focusing cameras. In these methods, distance of a scene from a camera is assumed a leg of a right triangle. PCT Publication WO2006120146 describes a triangulation auto focus method and system for focusing a camera in which a “spot beam emitter” marks a scene being imaged by the camera with a spot of light by illuminating the scene along a direction parallel to the camera axis. Displacement of an image of the spot in an image of the scene from a center of the image is used to provide triangulation data for determining distance to the scene.
U.S. Pat. No. 6,028,672 describes a “Rainbow Stereo 3D Camera” that exploits “projected color light with a spatially distributed wavelength spectrum on the surface of objects” in a scene to determine distances to the objects. “Multiple color imaging sensors separated by a baseline distance are used to capture stereo pair images of the scene at camera's frame rate. The 3D depth values are calculated using triangulation . . . ”.
US Patent Publication 2007/0091175 describes a gated time of flight camera for determining distances to features in a scene.
The disclosures of all of the above referenced patents and publications are incorporated herein by reference.